Cardiovascular System: The Heart and Blood Vessels – Study Guide

Chapter 18: The Cardiovascular System – The Heart

Heart Coverings and Wall Structure

The heart is protected and supported by several coverings and is composed of three main layers, each with distinct structure and function.

• Pericardium: A double-walled sac surrounding the heart, consisting of:

Fibrous pericardium: Tough, dense connective tissue that protects, anchors, and prevents overfilling.

Serous pericardium: Thin, two-layered membrane (parietal and visceral layers) with serous fluid in between to reduce friction.

• Heart Wall Layers:

Epicardium: Outer layer, also known as the visceral layer of the serous pericardium.

Myocardium: Middle, muscular layer responsible for contraction; composed of cardiac muscle cells.

Endocardium: Inner layer of endothelium lining the heart chambers and valves.

Heart Chambers and Associated Great Vessels

The heart has four chambers: two atria (upper) and two ventricles (lower), each with specific functions and associated vessels.

Right Atrium: Receives deoxygenated blood from the superior and inferior vena cava and the coronary sinus.

Right Ventricle: Pumps blood into the pulmonary trunk toward the lungs.

Left Atrium: Receives oxygenated blood from the four pulmonary veins.

Left Ventricle: Pumps oxygenated blood into the aorta for systemic circulation.

Blood Flow Pathways

Blood flows through the heart in a specific sequence, involving pulmonary, systemic, and coronary circuits.

Pulmonary Circuit: Right ventricle → pulmonary trunk → lungs → left atrium.

Systemic Circuit: Left ventricle → aorta → body tissues → right atrium.

Coronary Circulation: Supplies the heart muscle itself via coronary arteries and veins.

Heart Valves: Location, Function, and Operation

Valves ensure unidirectional blood flow through the heart.

• Atrioventricular (AV) Valves:

Tricuspid valve: Between right atrium and right ventricle.

Bicuspid (mitral) valve: Between left atrium and left ventricle.

Function: Prevent backflow into atria during ventricular contraction.

• Semilunar (SL) Valves:

Pulmonary valve: Between right ventricle and pulmonary trunk.

Aortic valve: Between left ventricle and aorta.

Function: Prevent backflow into ventricles after contraction.

Cardiac Muscle: Structure and Function

Cardiac muscle is specialized for continuous rhythmic contraction.

Striated, branched cells connected by intercalated discs (containing gap junctions and desmosomes).

Involuntary control and autorhythmicity (some cells can generate their own action potentials).

Differences from skeletal muscle: Shorter cells, single nucleus, more mitochondria, and longer refractory period.

Action Potentials in Cardiac Cells

Cardiac pacemaker and contractile cells have distinct action potential profiles.

Pacemaker Cells: Exhibit a prepotential (slow depolarization) due to Na+ influx, leading to spontaneous action potentials.

Contractile Cells: Action potential has a plateau phase due to Ca2+ influx, prolonging depolarization.

Absolute Refractory Period: Longer in cardiac muscle, preventing tetanus.

Key ions: Na+, Ca2+, K+

Intrinsic Conduction System

This system coordinates the heart's rhythmic contractions.

Components: SA node → AV node → AV bundle (Bundle of His) → bundle branches → Purkinje fibers.

AV Node Delay: 0.1 second delay allows atria to contract before ventricles.

Heart Block: Impaired conduction between atria and ventricles.

Electrocardiogram (ECG) Waves and Intervals

An ECG records the electrical activity of the heart.

P wave: Atrial depolarization.

QRS complex: Ventricular depolarization (and atrial repolarization).

T wave: Ventricular repolarization.

Abnormalities: Fibrillation (uncoordinated contraction), junctional rhythm (SA node failure).

Cardiac Cycle: Timing and Events

The cardiac cycle consists of all events associated with blood flow through the heart during one heartbeat.

Systole: Contraction phase.

Diastole: Relaxation phase.

• Phases:

Ventricular filling: AV valves open, blood flows into ventricles.

Isovolumetric contraction: All valves closed, ventricles contract.

Ventricular ejection: SL valves open, blood ejected.

Isovolumetric relaxation: All valves closed, ventricles relax.

End Diastolic Volume (EDV): Volume in ventricle at end of filling.

End Systolic Volume (ESV): Volume remaining after contraction.

Heart Sounds

Heart sounds are produced by valve closures during the cardiac cycle.

"Lub" (S1): Closure of AV valves at start of ventricular systole.

"Dub" (S2): Closure of SL valves at start of ventricular diastole.

Cardiac Output, Stroke Volume, and Heart Rate

Cardiac output is the amount of blood pumped by each ventricle per minute.

Formula: $CO = SV \times HR$

Stroke Volume (SV): $SV = EDV - ESV$

Cardiac Reserve: Difference between resting and maximal CO.

Regulation of Stroke Volume and Heart Rate

Multiple factors influence stroke volume and heart rate.

Stroke Volume: Influenced by preload, contractility, and afterload.

Heart Rate: Influenced by autonomic nervous system, hormones, and other factors.

Autonomic Nervous System and Heart Rate

The autonomic nervous system modulates heart rate via sympathetic and parasympathetic pathways.

Sympathetic stimulation: Increases heart rate and contractility.

Parasympathetic (vagal) stimulation: Decreases heart rate (vagal tone).

Homeostatic imbalances: Tachycardia, bradycardia, arrhythmias.

Chapter 19: The Cardiovascular System – Blood Vessels

Blood Vessel Wall Structure

Most blood vessels have three layers (tunics), each with specific functions.

Tunica intima: Endothelial lining, reduces friction.

Tunica media: Smooth muscle and elastic fibers, controls vasoconstriction and vasodilation.

Tunica externa (adventitia): Connective tissue, protects and anchors vessels.

Vasoconstriction and Vasodilation

Vasoconstriction: Narrowing of blood vessels due to contraction of smooth muscle.

Vasodilation: Widening of blood vessels due to relaxation of smooth muscle.

Types of Arteries

Arteries are classified by size and function.

Elastic arteries: Largest, closest to the heart (e.g., aorta); act as pressure reservoirs.

Muscular arteries: Distribute blood to organs; thick tunica media (e.g., brachial artery).

Arterioles: Smallest arteries; control flow into capillary beds; called resistance arteries.

Capillary Beds and Types of Capillaries

Capillaries are the site of exchange between blood and tissues.

Continuous capillaries: Most common; tight junctions; found in skin, muscle, brain.

Fenestrated capillaries: Pores for increased permeability; found in kidneys, intestines.

Sinusoidal capillaries: Large gaps; found in liver, bone marrow, spleen.

Capillary beds: Networks of capillaries; blood flow regulated by arterioles and precapillary sphincters.

Veins: Structure and Function

Veins return blood to the heart and differ structurally from arteries.

Thinner walls, larger lumens than arteries.

Valves prevent backflow.

Function as blood reservoirs.

Vascular Anastomoses

Vascular anastomoses are interconnections between blood vessels, providing alternate pathways for blood flow.

Blood Flow, Blood Pressure, and Resistance

Blood flow: Volume of blood flowing through a vessel, organ, or circulation per unit time.

Blood pressure: Force per unit area exerted on vessel wall by blood.

Resistance: Opposition to flow; mainly from friction in vessels.

Relationship: $F = \frac{\Delta P}{R}$ (Flow equals pressure difference divided by resistance)

Sources of Peripheral Resistance

Blood viscosity (thickness)

Vessel length

Vessel diameter (most influential)

Blood Pressure in Different Vessels

Highest in arteries, lowest in veins.

Steepest drop in arterioles.

Systolic pressure: Peak pressure during ventricular contraction.

Diastolic pressure: Lowest pressure during ventricular relaxation.

Pulse pressure: Difference between systolic and diastolic pressures.

Mean arterial pressure (MAP): Average pressure in arteries; $MAP = diastolic\ pressure + \frac{1}{3}(pulse\ pressure)$

Muscular and respiratory pumps: Aid venous return by compressing veins and creating pressure gradients.

Velocity of Blood Flow and Vasomotion

Velocity is fastest in arteries, slowest in capillaries, increases in veins.

Vasomotion: Intermittent flow of blood through capillaries due to precapillary sphincter activity.

Cross-sectional area: Inversely related to velocity; highest in capillaries.

Regulation of Blood Pressure

Main factors: Cardiac output, peripheral resistance, blood volume.

Factors increasing MAP: Increased heart rate, stroke volume, blood viscosity, vessel length, vasoconstriction, blood volume.

Capillary Exchange and Bulk Flow

Exchange of substances across capillary walls occurs by diffusion and bulk flow.

Hydrostatic pressure: Pushes fluid out of capillaries (filtration).

Colloid osmotic pressure: Pulls fluid into capillaries (reabsorption).

Filtration predominates at arterial end; reabsorption at venous end.

Vessel Type	Pressure	Velocity	Cross-sectional Area
Arteries	High	Fast	Low
Capillaries	Low	Slowest	Highest
Veins	Lowest	Increases	Low

Example: In the systemic circuit, blood leaves the left ventricle via the aorta (high pressure, fast velocity), slows down in the capillaries (allowing exchange), and returns to the right atrium via veins (low pressure, increased velocity due to larger lumen and valves).

Additional info: For more detailed mechanisms and clinical correlations, refer to Figures 19.10 and 19.13 in your textbook.